Watery exoplanet may be diamond-rich instead

Last year astronomers reported the discovery of a diamond planet orbiting a pulsar, a dead star releasing regular radio waves. Now another group says a previously discovered planet, 55 Cancri e, may also contain diamond - and this one orbits a more sun-like star.

Detected in 2004 via the wobbles it induces in its host star, 55 Cancri e already looked unusual. Previous observations led to the suggestion that it was covered in a deep ocean of supercritical water, a kind of water that blurs the line between liquid and gas. But that reasoning only works if you assume the planet is oxygen-rich like Earth, says Olivier Mousis at the University of Franche-Comté in Besançon, France.

Mousis and colleagues reckon this assumption is unsupported because the planet's star, 55 Cancri, is richer in carbon than it is oxygen. Recalculating with that in mind, taking the planet's mass and radius into account, suggests that about a third of that mass is carbon.

Based on estimates of the temperature and pressure throughout the planet, this carbon could be in the form of diamond, which requires extreme conditions to form. One of five known planets in its solar system, 55 Cancri e, sits very close to its host star. "When you form diamond, it's only a matter of temperature and pressure, and the temperature is very high on the surface," says Mousis.

The idea that 55 Cancri e is part diamond is still just that, and could yet be revised by further evidence of the planet's true composition. But one thing seems clear: this planet is no bore.